Golang文件操作深度解析：O_APPEND模式下的Seek行为与OS级特性

package main

import (
    "io"
    "log"
    "os"
    "strings"
)

func main() {
    filePath := "test_append.txt"

    // 确保文件存在，并写入一些初始内容
    initialContent := "Hello, World!\n"
    err := os.WriteFile(filePath, []byte(initialContent), 0666)
    if err != nil {
        log.Fatalf("Error writing initial file: %v", err)
    }
    log.Printf("Initial file content: %s", initialContent)

    // 1. 使用 O_APPEND 模式打开文件
    // 预期行为：Seek 无效，始终追加到末尾
    fileAppend, err := os.OpenFile(filePath, os.O_RDWR|os.O_APPEND, 0666)
    if err != nil {
        log.Fatalf("Error opening file with O_APPEND: %v", err)
    }
    defer fileAppend.Close()

    // 尝试 Seek 到文件开头
    _, err = fileAppend.Seek(0, io.SeekStart)
    if err != nil {
        log.Fatalf("Error seeking with O_APPEND: %v", err)
    }
    log.Println("Attempted to seek to start with O_APPEND.")

    // 写入新内容
    contentToAppend := "This should append.\n"
    _, err = io.CopyN(fileAppend, strings.NewReader(contentToAppend), int64(len(contentToAppend)))
    if err != nil {
        log.Fatalf("Error writing with O_APPEND after seek: %v", err)
    }
    log.Printf("Wrote '%s' with O_APPEND. Checking file content...", contentToAppend)

    // 读取文件内容验证
    finalContentAppend, err := os.ReadFile(filePath)
    if err != nil {
        log.Fatalf("Error reading final file: %v", err)
    }
    log.Printf("File content after O_APPEND write: \n%s", string(finalContentAppend))
    // 预期输出将是 "Hello, World!\nThis should append.\n"
    // 即使 Seek(0, io.SeekStart) 被调用，写入仍然在末尾发生。

    // ---------------------------------------------------------------------------------

    // 重置文件内容以便下一个测试
    err = os.WriteFile(filePath, []byte(initialContent), 0666)
    if err != nil {
        log.Fatalf("Error resetting file: %v", err)
    }
    log.Println("\n--- Resetting file for non-O_APPEND test ---")

    // 2. 不使用 O_APPEND 模式打开文件
    // 预期行为：Seek 有效，从指定位置写入
    fileNoAppend, err := os.OpenFile(filePath, os.O_RDWR, 0666)
    if err != nil {
        log.Fatalf("Error opening file without O_APPEND: %v", err)
    }
    defer fileNoAppend.Close()

    // 尝试 Seek 到特定位置 (例如，"World!" 的 'W' 之前)
    // "Hello, World!\n"
    // 01234567890123
    // Seek 到 7
    seekPos := int64(len("Hello, "))
    _, err = fileNoAppend.Seek(seekPos, io.SeekStart)
    if err != nil {
        log.Fatalf("Error seeking without O_APPEND: %v", err)
    }
    log.Printf("Attempted to seek to position %d without O_APPEND.", seekPos)

    // 写入新内容，覆盖部分原有内容
    contentToOverwrite := "GoLang"
    _, err = io.CopyN(fileNoAppend, strings.NewReader(contentToOverwrite), int64(len(contentToOverwrite)))
    if err != nil {
        log.Fatalf("Error writing without O_APPEND after seek: %v", err)
    }
    log.Printf("Wrote '%s' without O_APPEND. Checking file content...", contentToOverwrite)

    // 读取文件内容验证
    finalContentNoAppend, err := os.ReadFile(filePath)
    if err != nil {
        log.Fatalf("Error reading final file: %v", err)
    }
    log.Printf("File content after non-O_APPEND write: \n%s", string(finalContentNoAppend))
    // 预期输出将是 "Hello, GoLang!\n"
}